drm/i915/selftests/perf: measure memcpy bw between regions

Measure the memcpy bw between our CPU accessible regions, trying all
supported mapping combinations(WC, WB) across various sizes.

v2:
    use smaller sizes
    throw in memcpy32/memcpy64/memcpy_from_wc

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129093343.194570-1-matthew.auld@intel.com